@Swen so far this is indeed the only possible way. Be aware that you are not using https, and sending your credentials in plaintext over the internet (through a 3rd party service too). Downside is also that it does not understand natural speech, but only your predefined sentences.
@michbeck100 that looks good, I will test it when I get back home from holidays, but since Assistant supports Wemo, it should work.
I will also try to make time to start working on Smart Home for Actions on Google, but if any other dev has the time and knowledge to do this, please do so